In some applications utilizing power amplifiers, there is a need to vary the output power delivered to a load. For example, in a cell phone environment, it is desired to vary the output power of the cell phone based on various factors. For example, a base station may dictate the power level at which each cell phone should transmit (based on factors such as the physical distance from the base station, for example).
A standard method of controlling the output power of a power amplifier is to use a voltage regulator to regulate the battery or power supply voltage. Typical approaches to controlling the output power of a power amplifier use an “open loop” or a “closed loop” control technique. Closed loop techniques use an RF sensor, such as a directional coupler, to detect the power amplifier output power. The detected output power is used in a feedback loop to regulate the output power. “Open loop” techniques control the output power by regulating either the power supply voltage or power supply current used by the power amplifier. Open loop techniques are popular since open loop techniques do not have the loss and complexity associated with RF sensor elements.
Open loop techniques have several problems. For example, because output sensing is not used, components using open loop techniques suffer from inaccuracies and part-to-part variations. It would be desirable to use an open loop technique which achieves low thermal and minimal part-to-part variation.
FIGS. 1 and 2 show two prior art examples of open loop techniques for regulating the output power of a power amplifier. FIG. 1 is a schematic diagram showing an open loop voltage regulation technique. FIG. 1 shows a power amplifier 100 and a voltage regulator 102. The voltage regulator 102 is comprised of switching device M1, op-amp 104, and a feedback loop. The voltage regulator 102 provides a regulated voltage to the power amplifier 100, based on the voltage sensed and a desired power level indicated by an automatic power control signal VAPC. FIG. 2 is a schematic diagram showing an open loop current regulation technique. Like FIG. 1, FIG. 2 shows a power amplifier 100 and a voltage regulator 102. The voltage regulator 102 is comprised of switching device M1, op-amp 104, a current sense resistor R1, and a feedback loop. The voltage regulator 102 provides a regulated current to the power amplifier 100, based on the current sensed through resistor R1 and a desired power level indicated by power control signal VAPC.
In general, voltage regulation is preferred in an open loop design since voltage regulation does not suffer from the loss associated with a current sensing element, such as resistor R1 in FIG. 2. However, in many power amplifier designs, and especially in CMOS designs, voltage regulation can result in large thermal and part-to-part variations. These problems are caused in part by thermal voltage (VT) variations in transistors, which result in large power variations at low power, when the regulated voltage is close to the thermal voltage (VT) limit.